Earth-like?: Artist’s impression of the extrasolar planet Kepler-62f. (Courtesy: NASA Ames/JPL-Caltech/T. Pyle )Ten billion – that is the estimated number of Earth-like planets in orbit around Sun-like stars in the Milky Way. In One of Ten Billion Earths: How we Learn About our Planet’s Past and Future from Distant Exoplanets, astrophysicist Karel Schrijver takes a detailed look at what we know about exoplanets, and what this rapidly growing body of knowledge tells us about the Earth and its place in the solar system.We have known about exoplanets for about 25 years and Schrijver points out that the earliest discoveries were made in the most unlikely places. In 1992, for example, the first ever confirmed observation of an exoplanet involved an object about four times the mass of Earth orbiting a neutron star. Rather than surviving the supernova explosion that preceded the formation of the neutron star, Schrijver explains that the exoplanet and its two subsequently discovered companions were probably formed from the remnants of the explosion. So even in death, a star can acquire exoplanets.Another early surprise with possible implications for our solar system is the existence of “hot Jupiters” – gas giants that orbit much closer ...